Various medical applications, for example during a surgical procedure and within a certain time period thereafter, a patient's body requires to be externally heated, and preferably also uniformly heated. Heat transfer devices for transferring heat to the patient's body, which are used at present in operating rooms, are generally of two types: the first type utilizes a closed circuit with heated water circulating thereinside, and the second utilizes an open circuit, wherein heated air blows around the patient's body.
The heating technique of the first kind involves covering a patient with a thin rubber coves with warm water circulating through the cover. A device, equipped with a heat exchanger and pump, warms and circulates the water. Some of these devices also have the capability of regulating the temperature of the water based on core temperature measurement. These systems, however, give off heat to the environment and cause discomfort to the medical team. Moreover, these systems suffer from the following drawbacks: the very high cost of warming units, occasional leaking as the result of cutting the unit, blocking of the system when a patient is too heavy, and the system being bulky and heavy (containing about 3.5-5 liters of water).
As for the technique of the second type, it utilizes force air warming. These systems can be mostly preset at one of three thermal levels up to 43° C. They have different configurations to fit the upper body, lower body, torso, and full body, as well as pediatric blankets. The warm air in the system escapes through the pores on the upper part of the blanket as well as through the flexible tubes into the theater area, affecting the operating team and causing a certain degree of inconvenience. The energy suppliers can operate only one warming blanket at a time. Therefore, in most surgeries, either only the lower or the upper part of the body can be warmed by the system.
Intraoperative warming is needed during most operative procedures lasting longer than 30 minutes, and in all operations on children weighing less than 15 kilograms. The only situation where warming is practically not needed, are short procedures (less than 30 minutes), and when the temperature of the patient's body is intentionally lowered. Most patients begin losing body heat either prior to anesthesia or immediately upon the induction of anesthesia. Much body heat is lost within an hour of the start of anesthesia. Therefore, warming must begin either before or immediately following the induction of anesthesia.
One of the problems of intraoperative warming is associated with the requirements for servo-controlled delivery of heat, aimed at preventing overheating and allowing for the increased delivery of warmth to patients who are continuously losing body heat. Another problem is associated with the need for the circumferential warming of limbs, warming of discontinuous areas of the body and the possibility of warming sterile areas of the operative field. It is desirable to place a heating device on individual parts of the patient's body, so as to cover most of the body surface and to prevent the further loss of heat. Additionally, it is desirable to prevent the fan distribution of the warm air, and, while warming the desired parts of the patient's body, to prevent the warming of surgeons, nurses and anesthesiologists present at the time.
Comparing electrical heating to the above-described technique, the electrical heating has evident advantages, such as the possibility of the continuous control of heating, including computer control and automatic management.
U.S. Pat. No. 4,532,410 discloses an electrical heating device for providing a feeling of warmth to the entire human body for comfort and protection in active outdoors pursuits in cold environments by means of conventional, convenient, practical and desirable materials such as a neck scarf. A heater is disposed in an appendage or bib attached to and made as an integral part of the neck scarf. The heater is composed of a heating element made of Nichrome sandwiched between two covering elements made of a heat diffusible material such as triacetate tape.
The use of electrical heating in operating rooms needs to guarantee the following three complex tasks:
1. It should guarantee electrical safety, namely, completely stave off the damage of the electrical voltage, especially taking into account that various solutions are in use during an operation, the presence of various metal instruments, the patient is undressed, as well as the possibility of hemorrhaging.
2. It should guarantee to avert the danger of burns that could be caused by the local overheating of heating elements.
3. It should guarantee the absence of the influence of the electromagnetic effects on electronic equipment involved in the operation or located in the vicinity of the heating device.
It is important to understand that in most cases, the above three tasks have to be accomplished together and completely. For example, to avoid the damage of electrical voltage, voltages as low as possible must be used. However, the use of low voltages with the required power compels the operation to take place with a high current, while this increase in current might affect the electronics located in the vicinity of the beating device.
To decrease the influence of the electromagnetic field of an electric power source onto the functioning electronics, it is known to utilize a bifilar arrangement of conductors. This technique is disclosed for example, in the patent document DE29610436. However, this technique unavoidably leads to local heating, owing to the fact that the conductors in which heat required for heating an object (e.g., a patient's body) is dissipated, are located very close to each other.